Mobile networks communication continues to grow rapidly. The mobile data usage will continue skyrocketing. New data applications and services will require higher speed and more efficient. Large data bandwidth application continues to attract more consumers. Today, 3G/4G mobile wireless systems provide connectivity for wide range of applications and services. In the next generation 5G network, it is expected that the latency issue will be one of the key performance indicators (KPIs), which shape the air interface and network architecture design.
Short TTIs (Transmission Time Interval) are effective in reducing end-to-end latency and ultimately improves user experiences. In the long term evolve (LTE) system, typically eNB uses downlink control channel to inform UE of the downlink data transmission. One to three OFDM symbols in a subframe with fourteen OFDM symbols are set aside for control signaling, including PDCCH, PHICH, and PCFICH. Even if there is very little control signaling actually sent in a subframe, the whole control region is still reserved and the spare resource not taken by downlink control still cannot be used by downlink data, except that PCFICH can be used to control the size of the control region. On a short TTI, such a practice can lead to a severe waste of radio resources. Further, the data packets for one UE can be of different sizes and can have different latency requirements, which are best addressed with TTIs with different lengths. For a UE, if the network can support only one TTI length at any given time, then eNB cannot address the different requirements satisfactorily.
Improvements and enhancements are required to improve system latency. In particular, enabling resource sharing between downlink control and downlink data and simultaneous supports for TTIs of different lengths are desired.